1. Field of the Invention
The invention relates to a fuel injection pump, having a metering pump and an injection adjuster for the metering pump, in which the injection adjuster has a piston that is acted upon by a restoring spring.
2. Description of the Prior Art
One fuel injection pump is known from German Patent Disclosure DE 43 44 865 A1 is a so-called distributor injection pump, in which a single pump piston of the metering pump is used to inject the correct fuel quantity into the appropriate cylinder of an internal combustion engine. During the revolution of the drive shaft, the pump piston executes as many strokes as there are engine cylinders to be supplied; the drive shaft of the fuel injection pump rotates at half the rotary speed of the engine crankshaft.
Since the fuel need not always be injected into the applicable cylinders at the same instant, with reference to the angle of rotation of the crankshaft, an injection adjuster is provided, by means of which the instant of injection can be adapted to the prevailing operating conditions of the engine. In a distributor injection pump, this is done by adjusting a cam ring, which actuates the pump piston and is driven by the drive shaft, by approximately xc2x110xc2x0 relative to the drive shaft.
For adjusting the cam ring, the piston is provided, which is pressed by the restoring spring against an end stop. This position in contact with the end stop is equivalent to an injection adjustment in the xe2x80x9clatexe2x80x9d direction. In order to bring about the injection adjustment during operation, the side of the piston opposite the restoring spring is subjected to a fluid that is under pressure and that is controlled by a control slide and is furnished by a prefeed pump of the injection pump. As a consequence, however, when the fuel injection pump is at a stop, the piston of the injection adjuster is in the xe2x80x9clatexe2x80x9d position, since at a stop no fluid flow can be furnished, and thus only the force that is furnished by the restoring spring acts on the piston.
A disadvantage of this is that the position of the injection adjuster piston, which is predetermined by the restoring spring, at a stop is not optimal for starting an internal combustion engine supplied by the fuel injection pump.
The fuel injection pump of the invention offers the advantage that the starting spring acting counter to the restoring spring keeps the piston of the injection adjuster in the optimal position for starting the engine when the fuel injection pump is at a stop and consequently no fluid flow with which the piston could be acted upon is available. This optimal position, which is selected especially with a view to starting performance at low temperatures, can preferably be determined by adaptation of the prestressing and the spring constants of the restoring spring and the starting spring.
In a preferred embodiment, it is provided that the starting spring is disposed in the interior of the piston. This produces an especially compact structural shape.
In one embodiment, it is provided that the starting spring is braced by one end on the housing of the injection pump. In this way, a simple, open construction at little additional cost is obtained.
In an alternative embodiment, it is provided that the starting spring is braced by one end on an auxiliary piston, which is supported displaceably in a receiving bore in the piston and is braced on the housing. In this encapsulated version, only a relatively short spring is needed, and such a spring can be designed well.
To prevent the auxiliary piston from coming loose from the injection adjuster piston, a securing ring that limits the displaceability of the auxiliary piston can be disposed in the receiving bore.